The present invention is directed to the production of seamless tubing, and more particularly to the provision of new procedures and a new complex of mill equipment for the production of high quality seamless tubing on an economical basis, at relatively low production levels.
The production of seamless tubing is, in general, an old and well known art. Typically, a heated billet is pierced, either in a press piercing mill or by way of a cross roll piercer. The pierced billet then undergoes one or more elongation stages for increasing the length of the billet while reducing its wall thickness and adjusting its diameter. Typically, the principal elongation stage involves the use of a mandrel mill, Assel mill, plug mill or push bench for example. Of these, the mandrel mill is generally accepted as the most productive. However, the mandrel mill is an expensive installation and thus not easily adapted for the economical production of seamless tubing at relatively low production levels as contemplated by the present invention. The other mentioned elongation processes, on the other hand, while being less costly to install have certain quality limitations that tend to restrict the end use of the resulting product. For example, because of the relatively abrupt transitional change in the roll diameter of the Assel mill, the surface of the resulting tubular product often has a somewhat "crazed" appearance, making it relatively unacceptable for oil field use, for example, because of concerns over failure.
The present invention is directed to a new seamless tube process which, at relatively low capital cost, more or less competitive with Assel mill installations, for example, nevertheless enables a uniquely high quality of seamless tubing to be produced, without the limitations characteristic of the Assel mill. More specifically, pursuant to the process and installation of the invention, the primary elongator facility is a Diescher type mill provided with a restrained mandrel system. With this arrangement, it is possible in a seamless tube mill of so-called "mini-mill" proportions, to produce tubing of very high quality, in a wide range of sizes up to ten inches and more in diameter and in so-called "double length" sections, up to ninety-six feet in length.
Optimum quality and economy considerations are balanced in the procedure of the present invention by the use of so-called No. 1 and No. 2 cross roll piercers in advance of the Diescher type, restrained bar elongator. With this arrangement, it is possible to produce double length tubing to a high quality level in terms of concentricity and uniformity of wall thickness. Production of tubing in double length also provides for significant economies in terms of reduced crop end loss, as will be readily understood.
For a more complete understanding of the above and other features and advantages of the invention, reference should be made to the following detailed description of a preferred embodiment, and to the accompanying drawing.